1. Field of the Invention
The present invention relates to an improvement of a magnetic field generating device for use in a magnetic resonance imaging apparatus (hereinafter referred to as MRI) for medical treatment use and the like and, more in particular, it relates to an MRI magnetic field generating device intended for reducing eddy current in a pole piece and remanent magnetism by gradient magnetic field coils without deteriorating the uniformity of a magnetic field in the magnetic space thereby enabling high speed image pick-up by constituting a pair of pole pieces opposed at a space to each other as a laminate structure in which soft ferrite or a laminate silicon steel sheet and soft ferrite is disposed on a magnetic material base and interposing a layer of small magnetic permeability and high electric resistance between the soft ferrite and the magnetic material base or between the soft ferrite and the laminate silicon steel sheet.
2. Description of the Related Art
MRI is an apparatus capable of obtaining tomographic images of an object to visualize as far as the nature of tissues thereof by inserting a person to be diagnosed partially or wholly in a space of a magnetic field generating device that forms a strong magnetic field.
In the MRI magnetic field generating device, it is necessary that the space has such an extent as capable of inserting the person to be diagnosed partially or wholly, and it is required to form a stable and intense uniform magnetic field at 0.02 to 2.0 T and having accuracy of less than 1.times.10.sup.-4 (usually) in an image pick-up view field in the space in order to obtain clear tomographic images.
As a magnetic field generating device used for MRI, it has been known a constitution as shown in FIG. 6(A) and (B), in which a pair of permanent magnet constituents 1, 1 using R-Fe-B type magnets are opposed to each other with pole pieces 2 and 2 being secured to respective one ends (as a magnetic field generating source) and connected at the other ends with yokes 3, to generate a static magnetic field in a space 4 between the pole pieces 2 and 2 (Japanese Patent Publication Hei 2-23010). The figure shows an embodiment in which yokes 3 are formed with a pair of plate yokes 3a and 3b and four columnar yokes 3c.
For the pole pieces 2 and 2, an arrangement of providing an annular protrusion 5 at the circumference and further disposing a convex protrusion (not illustrated) at a central portion is adopted in order to enhance the uniformity of a magnetic field distribution in the space 4 (Japanese Utility Model Publication Hei 5-37446).
Further, as the magnetic field generation source, it has been known to replace the above-mentioned permanent magnet constituents 1 and 1 with electromagnetic coils (including, for example, ordinary conductive coils and superconductive coils) wound around the periphery of a core (not illustrated) (Japanese Patent Laid-Open Hei 4-288137), in which pole pieces similar to those in FIG. 6 are used.
It has also been known a magnetic field generating device which is based on the constitution as shown in FIG. 6 and in which four columnar yokes 3c are replaced with a C-shaped yoke where a pair of plate yokes 3a and 3b are connected only at one ends by a plate yoke 3d (Japanese Design No. 847566) as shown in FIG. 7.
Further, as shown in FIG. 8, there has been also known a magnetic field generating device in which columnar yokes 3c are disposed to both ends of a pair of plate yokes 3a and 3b (at a position on or behind an imaginary line passing through the center of a permanent magnet constituent 1 and in perpendicular to the direction of inserting a person to be diagnosed) (Japanese Patent Laid-Open Hei 6-78893).
The constitutions shown in FIG. 7 and FIG. 8 have a merit, as compared with the constitution shown in FIG. 6, of being free in the direction of inserting the person to be diagnosed into the space 4 and gives no oppressive sensation on the person. Also, in these constitutions, pole pieces similar to those in FIG. 6 are used.
In a proposed magnetic field generating device of another constitution, as shown in FIG. 9, permanent magnet constituents 11, 11 of rectangular section and permanent magnet constituents 12, 12, 12, 12 of triangular section each made of a R-Fe-B type magnet are disposed as a magnetic field generation source on an inner circumferential surface of a polygonal (hexagonal in the drawing) cylindrical yoke 10 and, in particular, pole pieces 13 and 13 are secured to space-facing faces of a pair of permanent magnet constituents 11 and 11 of rectangular section forming a main magnetic field, thereby generating a static magnetic field in a space 14 between the pole pieces 13 and 13 (Japanese Patent Laid-Open Hei 6-151160).
For enhancing the uniformity of a magnetic field distribution in the space 14, the pair of pole pieces 13 and 13 are provided at both axial ends thereof with protrusions 15 and 15 each in the shape of a rectangular bar and in a central portion thereof with a protrusion 16 in the shape of a rectangular plate.
As described previously, various constitutions have been known for the magnetic field generating device for use in MRI. In any of such constitutions, pole pieces are disposed on the space-facing surfaces of the permanent magnet constituents in order to enhance the uniformity of the magnetic field distribution in the space 4, and the pole pieces usually comprise a bulk cut out from a magnetic material such as electromagnetic soft iron or pure iron.
In the MRI, gradient magnetic field coils (GC) comprising three sets of coil groups corresponding to three directions X, Y and Z are usually arranged near each of pole pieces disposed to the space-facing surfaces of the permanent magnet constituents (the gradient magnetic field coils are simply indicated by 6 in FIG. 6,) and a gradient magnetic field in a desired direction can be generated within the space by applying a pulse current to the gradient magnetic field coils.
Namely, positional information is given to nuclear magnetic resonance signals by applying a gradient magnetic field to the uniform magnetic field formed in the space, and it is necessary to apply a plurality of pulsative gradient magnetic fields in order to obtain images.
For instance, an ideal waveform of a magnetic field generated by gradient magnetic field coils is conceptionally illustrated in FIG. 10(A), which has a rectangular waveform with an extremely short period of time to reach a predetermined magnetic field intensity (rise and fall time), and has a symmetrical shape in which a wave height is equal between positive (+) and negative (-) sides (B.sub.1 =B.sub.2). Further, the magnetic field intensity is reduced to 0 between adjacent rectangular waveforms.
However, since the pole pieces are disposed near the gradient magnetic field coils, when a pulse current is supplied to the gradient magnetic field coils, the magnetic field changes abruptly upon rise and fall of the pulse current to generate an eddy current in the pole pieces each comprising a bulk material made of electromagnet soft iron, pure iron or the like described above. In addition, the pole pieces are magnetized by the magnetic fields of the gradient magnetic field coils to disturb the uniformity of the magnetic field in the space due to magnetic hysteresis (remanent magnetism) in the pole pieces.
Generation of the eddy current or the remanent magnetism greatly disturbs the waveform of the magnetic fields generated by the gradient magnetic field coils.
Namely, as shown in FIG. 10(B), it requires much time to reach a predetermined magnetic field intensity (rise or fall time) and the wave height is different between the positive (+) and the negative (-) sides (B.sub.1 .noteq.B.sub.2) to form an asymmetric waveform. In the figure, a dotted line corresponds to an ideal waveform shown in FIG. 10(A). In addition, since this rectangular waveform is not an ideal waveform, the magnetic field intensity is not reduced to 0 between adjacent rectangular waveforms.
In recent years, since it has been required to pick-up further clear image at a high speed, a constitution of, for example, utilizing a pulse sequence of switching the gradient magnetic field at a high speed such as a FSE method (Fast Spin Echo) has often been adopted. However, when the waveform of the magnetic field generated by the gradient magnetic field coils shows an asymmetric waveform as shown in FIG. 10(B) due to the effect of the eddy current or the remanent magnetism, no aimed images can be obtained because of generation of signal error or the like.
The present applicant has proposed a MRI magnetic field generating device comprising various constitutions improved for pole pieces situated near the gradient magnetic field coils as means for dissolving the foregoing problems.
For instance, a pole piece 20 shown in FIG. 11 is adopted to the MRI magnetic field generating device, which is a pole piece comprising a so-called laminate structure having a constitution shown in FIG. 6, wherein a pure iron ring 22 of rectangular section is disposed at the periphery of a space-facing surface of a disc-shaped pure iron magnetic material base 21, and a plurality of block-shaped soft ferrite materials 23 are laid in a central portion (Japanese Patent Laid-Open Hei 4-23411).
In the drawing, a pure iron core 24 constitutes a substrate for mounting gradient magnetic field coils.
The applicant has further proposed a pole piece in which block-shaped laminate silicon steel sheet 26 comprising a plurality of silicon steel sheets laminated and integrated in a space-facing direction is laid as shown in FIG. 12 and FIG. 13, instead of the block-shaped soft ferrite 23 shown in FIG. 11 (Japanese Patent Laid-Open Hei 4-138131).
FIG. 12 shows block-shaped laminate silicon steel sheets 26 using directional silicon steel sheets in which a plurality of directional silicon steel sheets each having a directionality in one identical direction are previously laminated and integrated in the direction of the thickness to form sub-blocks 26a and 26b (arrow in the figure indicating the direction of axis of easy magnetization) and, subsequently, the sub blocks are laminated and integrated such that the directions of the axes of easy magnetization of the sub-blocks are in perpendicular to each other.
FIG. 13 shows block-shaped laminate silicon steel sheets 26 using non-directional silicon steel sheets in which they are merely laminated and integrated in the direction of the thickness irrespective of the directionality as in the constitution of FIG. 12.
The applicant has further proposed a pole piece 30 as shown in FIG. 14 in which the block-shaped laminate silicon steel sheets and block-shaped soft ferrite are effectively laminated and arranged (Japanese Patent Laid-Open Hei 5-182821).
That is, a pure iron ring 32 of rectangular section is disposed at the periphery of a space-facing surface of a disc-shaped pure iron magnetic material base 31, and block-shaped laminate silicon steel sheets 36 and a block-shaped soft ferrite 33 are laminate and arranged such that the laminate silicon steel sheets 36 abut against the magnetic material base 31 to form a pole piece.
FIG. 15 shows a pole piece 40 used for the MRI magnetic field generating device comprising the constitution as shown in FIG. 9, in which pure iron protrusions 42 each in the shape of a rectangular bar are disposed on both axial ends and block-shaped soft ferrite 43 is laid in a central portion on the space-facing surface of a magnetic material base 41 in the shape of a rectangular plate made of pure iron (Japanese Patent Laid-Open Hei 6-151160).
Also in this constitution, block-shaped laminate silicon steel sheets or the block-shaped laminate silicon steel sheets and the block-shape soft ferrite may be laminated and arranged in place of the block-shaped soft ferrite 43 at the central portion.
It is possible to reduce the eddy current and the remanent magnetism by the use of the pole piece having the soft ferrite or the silicon steel sheet arranged effectively, but a further improvement has still been demanded for MRI which adopts, for example, the FSE (Fast Spin Echo) method as explained previously.
That is, the demand for the improvement of the image clarity and shortening of the image pick-up time has been increased more and more, along which a pulse current applied to the gradient magnetic field coils tends to increase and the intensity of the magnetic field generated by the pulse current tends to increase further.
Although soft ferrite, having high electric resistance, is effective for reducing the generation of the eddy current, it tends to cause magnetic saturation due to a multiplier effect of the magnetic field by the magnetic field generating source and the magnetic field by the gradient magnetic field coils.
Therefore, also in the pole piece having the soft ferrite disposed to the space-facing surface, the magnetic flux density in the soft ferrite increases along with the increase of the magnetic field intensity by the gradient magnetic field coils to cause magnetic saturation partially or entirely, so that a portion of magnetic fluxes leaks to the magnetic material base or the laminate silicon steel sheet in contact with the magnetic saturation portion of the soft ferrite.
Accordingly, the leaked magnetic fluxes cause the eddy current and the remanent magnetism in the magnetic material base or the laminate silicon steel sheet.
For reducing the effect of the magnetic field by the gradient magnetic field coils on the magnetic material base or on the laminate silicon steel sheets, it may be considered to increase the thickness of the soft ferrite. However, this is not economical since the cost for the soft ferrite is increased, as well as increase for the thickness of the soft ferrite inevitably increases the entire thickness of the pole piece, which results in increase for the magnetic flux leakage from the outer circumferential surface of the pole piece to enlarge the scale of the device.
The laminate silicon steel sheets have higher saturation magnetic flux density (Bs) as compared with the soft ferrite and possess a merit of reducing the generation of the eddy current and easily attaining the enhancement for the uniformity of the magnetic field in the space. However, since the coercive force of the silicon steel is somewhat greater as compared with that of the soft ferrite, it is difficult to completely prevent the generation of the remanent magnetism to cause remanent magnetism at an order of several m Gauss, which disturbs uniformity, though little, of the magnetic field in the space particularly, in a pole piece in which the laminate silicon steel sheets are disposed on the space-facing surface.